The invention relates to a mass flow metering device which operates within a defined fluid stream. Such metering devices are desirably constructed without internal moving parts which may be contaminated by the fluid within the stream. The principle of the invention is based on the known fact that a fluid flowing through a conduit or tube which experiences an acceleration orthoginal to the direction of its flow, will interact with the conduit wall with a reaction force which is directly proportional to the mass flow of the fluid within the conduit. The reaction force generated by the fluid against the conduit is generally referred to as a Coriolis force.
Various issued patents describe mass flow meters which utilized the measurement of the fluid reaction forces to determine the mass flow rate. These patents teach various conduit designs and configurations, various means for measuring the reaction forces and various ways of determing the mass flow.
Roth, U.S. Pat. No. 2,865,201, teaches a gyroscopic type flow meter which directly measures the magnitude of the reaction forces on the conduit. Since these forces are created by a continuous oscillation of the conduit, the Roth design is impractical. Similar conduit designs are found in Roth, U.S. Pat. No. 3,276,257, and Henderson, U.S. Pat. No. 3,108,475. The sensitivity of the reaction force measurement in all of these conduit designs is greatly influenced by the oscillatory fluctuations of the meter conduit and by environmental vibrations.
A series of patents, U.S. Pat. Nos. 3,261,205, 3,329,019 and 3,355,944, to Sipin teach the measurement of the fluid reaction forces due to an imparted transverse vibration on a straight conduit, a curved conduit and a U-shaped conduit. The earlier conduit designs in this series attempt to directly measure the reaction forces on the conduit and, therefore, were subject to the same substantial sensitivity deficiencies due to external vibrational influences found in the patents discussed above. In the curved and U-shaped conduit designs, the imparted oscillation creates a torsional bending moment about an, ideally, fixed axis. In the U-shaped design the sensors were required to be referenced to the actual motion of the tube and to a fixed or stationary position. In a working environment each of the Sipin conduit designs are extremely noisy in operation and, basically, ineffective due to inacuracies created by vibrations of the flow meter and the references of the sensors tube unrelated to the fluid reaction force. The drivers, which impart the oscillatory motion to the conduit, are attached to an external casing of the meter. The internal and external vibrational effects causes substantial output deficiencies in the reaction force sensing means and, therefore, greatly effect the calculation of the mass flow rate.
In Smith, U.S. Pat. No. 4,109,524, an attempt was made to separate the oscillation means from the force measurement system. The flow meter disclosed in this patent is cumbersome in application and does not effectively reduce the vibrational effects on the reaction force sensing means.
The first patent to recognize the need for vibrational and noise immunity on the sensing means is Cox et al, U.S. Pat. No. 4,127,028. In Cox each reaction force sensor is referenced to two adjacent cantalevered tubes. The two tubes are oscillated simultaneously in opposite relative directions and, ideally, at the same resonance. The external vibrational influences on the two tubes are intended to be self-cancelling when viewed by the sensors referenced to both tubes. However, the driving means in this design is mounted on a long cantilever arm and includes a large weight at the end of the arm. This structure produces an extremely low vibrational resonance and greatly limits the ability of the cantalevered tube to oscillate about a fixed reference axis. Environmentally induced vibrations, as well as vibrational effects of the driving means continue to influence the Cox measurement sensitivity by affecting the positioning of the tubes differently.
The same deficiencies found in Cox '028 in its reaction force sensing are found in the Smith, U.S. Pat. No. 4,187,721 and its corresponding Reissue No. 31,450. Smith, U.S. Pat. No. 4,422,338, attempts to enhance the sensitivity of the meter by using a frame which surrounds the oscillating tube to act as a fixed sensor reference. In addition, the Smith '338 design utilizes velocity type sensors to create an adjoining reference system such that the zero or reference position of linear type sensors, which record the tube motion due to the fluid reaction forces, is continually adjusted in response to vibrational influences on the meter. However, since the rotational axis of the cantalevered flow meter tube and mounting frame is not stationary, due to the vibrational effects on the meter structure. The effect of adjusting the reference plane of the reaction force sensors, therefore, is minimal. Commonly assigned copending application Ser. No. 809,659 submitted to the Patent Office on Dec 16, 1985 teaches a conduit design which is not cantilevered and is driven preferably directly along the axial line of the pipeline of the defined fluid stream. The structure of this invention overcomes many of the prior art deficiencies in sensing.
It is important to note that in all of the known flow meter designs, as long there is an increasing gradient of transverse velocity from the entrance of the flow meter tube to a point of maximum velocity and a decreasing transverse velocity gradient from the maximum point to the outlet, that there will be a decreasing transverse reaction or Coriolis force gradient in one direction from the inlet to the point of maximum deflection or velocity and a transverse force gradient in the opposite direction from the maximum point to the outlet. The measurement or sensing of these reaction forces created by the Coriolis reaction of the fluid maybe correlated to the mass flow rate within the tube.
The prior art of this type flow meter exhibits significant deficiencies in their determination of the fluid reaction on the tube. These deficiencies are directly related to the geometry of the meter and its sensing technique. It is difficult to isolate the oscillating motion of the flow meter tube created by the fluid reaction forces due to the environmental vibrations encountered by the conduit (or vibrations created by the meter itself).
The typical industrial environment in which the flow meter operates is subject to substantial vibrational influences due to the presence of rotating machinery within the process line in which the meter is located. External temperature influences, as well as, internal pressure and temperature fluctuations adversely affect the reliability and the sensitivity of the known meter designs.
Additional problems which effect the sensitivity of this type flow meter relate to the utilization of these instruments "on line" within an existing piping system in an industrial process. Impedance of the fluid flow caused by the flow meter may significantly hamper the efficiency of the industrial process.
Furthermore, flow meters of this type have a tendency to become complex, bulky and expensive, all of which adversely affect the applicability of the Coriolis or gyroscopic measurement technique in many instances.